vickij,
We can help you with the terminology. A "differential" is a test of your white cells to see how many are this type or that type. The types are usually called neutrophils, eosinophils, basophils, lymphocytes, and monocytes but are often abbreviated. The lab can do a differential test along with your regular blood tests if the doctor asks for it.
The differential numbers let the doctor (and you if you want to) compute a number called your
Absolute Neutrophil Count (ANC), which is a rating of how susceptible to infections you are.
5q minus syndrome, also called MDS 5q-, is one of the "better" chromosome problems to have, if you can call anything about this disease "better", because it means that you are very likely to respond well to treatment with lenalidomide, which goes by trade name Revlimid in the U.S.
Having 5q- also makes it pretty clear that you have MDS, not aplastic anemia. I'd be surprised if they conclude that a repeat of ATG treatment is a better choice than Revlimid, but the doctors know more than us and sometimes surprise us.
Having your sister tested certainly makes sense. There's only a 25% chance that a sibling will be an exact match, but I hope that's the case.
I suggest that you keep ongoing notes in a notebook or on your computer. It helps to take notes during appointments (especially if someone else can come along to take them!) so you can remember what was discussed and any instructions the doctor gave you. Too often, appointments rush by and then we say "wait, what did he/she say?" to ourselves.
If you ask for copies of your CBCs (complete blood count tests) you can track your blood counts over time, and you'll see the differential numbers show up on the report if the doctor ordered that test. You don't have to keep track of every word everyone says, just the main points, but the longer you're at it the more it'll start to make sense.
Anytime one of my wife's doctors mentions a medicine I don't recognize, I interrupt them and ask how to spell it. That way I can look it up after the appointment and learn what it's for and what side effects it has. A lot of drugs have scientific names (e.g, lenalidomide) and brand names (e.g., Revlimid) and as you read you end up learning both. Search for either word in these forums and you'll find discussion about them.